There are many circumstances in which it is necessary to effectively operate an apparatus for a process for separating particulate material from a high temperature gas. For example in circulating fluidized bed combustion or gasification processes, large amounts of dust (which may include solid bed material, ashes, unburned fuel, reactive absorbents, etc.) are continuously entrained with the exhausted high temperature gases from the combustion chamber. In some cases, the dust is recirculated back into the combustion chamber after the particles have been separated. In this, and many other embodiments, gas permeable high temperature filters may be utilized. Such filters typically are a plurality of elongated parallel tubes of porous super alloys or ceramics or combinations thereof that can endure high temperature conditions and do not need to be protected by cooling surfaces or refractory linings. However when such filters are utilized, dust gradually accumulates on the surfaces of the porous material and the filter has to be periodically cleaned.
Cleaning of such filters cannot be done by shaking, vibrating, or even brushing or scraping, since such filters may be very fragile and consequently easily damaged by rough treatment. Therefore, typically such porous filter tubes are cleaned by a high pressure backflushing pulse of cleansing gas. For example U.S. Pat. No. 4,161,389, and co-pending application Ser. No. 07/378,628 filed Jul. 12, 1989, teach the utilization of backflushing pulses of cleansing gas to clean elongated porous filter elements.
In a typical system for supplying a pulse of cleansing gas to the filter elements, a quick acting valve is disposed between a source of cleansing gas under high pressure, and the clean side of the filters, operation of the valve by a solenoid or the like causing a pulse of gas to pass from the high pressure source through the porous filter elements. Since only a pulse of gas is provided, there is typically no adverse thermal shock to the fragile filter elements at high temperature (the cleansing gas is necessarily at significantly lower temperatures than the gas from which the particulates are being separated), and does not interfere with the normal passage of the "dirty" gas into operative association with the filter elements, the backflushing action taking a very small period of time. Unfortunately, however, quick acting valves are delicate structures, and can malfunction for many different reasons. If they do malfunction, a large volume of gas can pass from the source into contact with the filter elements--rather than merely the designed "pulse" of gas sufficient only to effect dislodgement of collected particles on the filtering element walls, without thermal shock. If such a large volume of gas passes from the high pressure but at a low temperature source of cleansing gas through the filter elements, that can cause thermal shock, and thereby breakage or leakage of the filter elements, and additionally it interferes with the filtering process, and causes one to shut down the entire process--which may include a circulating bed reactor in association with the filtering elements.
According to the present invention, the above described problems have been remedied by positively controlling the volume of gas supplied to the filter elements. Positive control is accomplished by providing a reservoir of cleansing gas having a volume sufficient to effect particle dislodgement from the filtering elements, but insufficient to cause thermal shock or major interruption of the filtering process, with a flow rate restrictor disposed between the source and the reservoir. The restrictor allows only a small flow rate of gas therethrough, so that the volume of the reservoir fills up between the periodic actuations of the quick acting valve, but so that the flow rate is insufficient to cause thermal shock to the filtering elements. Of course the exact size of the restrictor opening will depend upon the temperatures and flow rates of the gases, the particular materials of the filter elements, the volumes of the reservoirs, etc. The restrictor typically comprises an orifice or a nozzle having a diameter measured most effectively in millimeters, although--again--the dimensions will vary widely depending upon the particular operational and equipment parameters.
According to one aspect of the present invention, an apparatus is provided comprising: (a) A vessel having a dirty gas inlet, a separated particulate material outlet, and a clean gas outlet. (b) A plurality of filter elements having filter pores therein which allow the passage of the high temperature gas therethrough, but filter out the majority of the particulates contained in the gas, the dirty gas inlet being provided on one side of the filter elements, and the clean gas outlet being provided on the other side of the filter elements. And, (c) means for supplying a positively controlled volume, high speed, cleansing gas pulse to the filter elements to effect removal of particles which might collect thereon.
The means (c) preferably comprises the following elements: A source of high pressure cleansing gas. A reservoir of high pressure cleansing gas operatively connected to the source by a gas flow rate restrictor means. A conduit connected between the reservoir and the clean side of the filter elements. And, a quick acting valve disposed in the conduit.
As earlier described, the filter elements preferably are elongated tubular elements, which can be open throughout the entire middle portion thereof to the dirty gas flow, or can have one closed end and be opened to the exterior opposite to the closed end thereof to the dirty gas flow. If desired, the clean side of the filters may be divided into a plurality of different chambers, with one reservoir, and a restrictor associated with each chamber. Also, a number of different reservoirs and restrictors can be associated with the same chamber or the entire clean side of the filter, and the appropriate reservoir will be selected depending upon temperature and flow conditions, or the like.
The apparatus according to the invention--in one exemplary particularly worthwhile utilization thereof--is utilized in a circulating fluidized bed reactor having a downstream filter housing and reactor chamber arranged back-to-back one with the other, such as disclosed in U.S. Pat. No. 4,869,207, the disclosure of which is hereby incorporated by reference herein.
According to another aspect of the present invention, a method for removing particulate material from high temperature gas is provided. The method comprises the steps of: (a) Feeding dirty gas into contact with the dirty side of the filter element. (b) Withdrawing clean gas from the clean side of the filter element. (c) Periodically supplying a high pressure pulse of cleansing gas to the clean side of the filter elements to dislodge particles which have adhered to the dirty side of the filter elements, the cleansing gas having a lower temperature than the high temperature gas from which particulates are being separated. And, (d) positively controlling the volume of the cleansing gas pulse in step (c) to minimize thermal stock to the filtering elements and to minimize interruption of the filtering action effected when previously dirty gas passes through the filtering elements. Step (d) is preferably accomplished by: Providing a high pressure source of cleansing gas. Providing a reservoir of a controlled volume, for the cleansing gas. And, restricting the passage of cleansing gas between the source and the reservoir so that the flow rate of gas is insufficient to cause thermal shock.
Steps (c) and (d) are preferably practiced by placing the reservoir in operative communication with the clean side of the filter elements to effect cleansing action. Step (c) is also preferably practiced by supplying cleansing gas at approximately sonic velocity.
It is the primary object of the present invention to provide for the effective and non-destructive cleansing of filter elements during the separation of particulates from high temperature gas. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.